Castel Sant'Elmo is a medieval fortress located on a hilltop near the Certosa di San Martino, overlooking Naples, Italy. The name "Sant'Elmo" derives from a former 10th-century church, Sant'Erasmo, shortened to "Ermo" and, finally altered to "Elmo". It presently serves as a museum, exhibition hall, and offices.
Documents date a structure at the site from 1275, from the era of Charles d'Anjou. Known originally as Belforte, it was likely a fortified residence, surrounded by walls, its entrance gate marked by two turrets. In 1329, using designs by the Sienese architect Tino da Camaino, the Duke Robert d'Anjou enlarged the fortress described in documents as palatium in summitatae montanae Sancti Erasmi. Camaino also supervised construction of the adjacent Carthusian monastery of San Martino. By 1336, the palace was referred to as a castrum or castle, and work continued under Camaino till his death in 1343.
The Angevin fortress was severely damaged in an earthquake in 1456, which demolished the external walls and the towers. The Argonese rulers of Naples, and notably Don Pedro de Toledo, the first governor and cousin of the Viceroy, included it in a comprehensive scheme designed to fortify the land perimeter of the city, based on four separate strongholds. Castel Sant'Erasmo acquired its hexagonal star shape between 1537 and 1547 under the designs of Pedro Luis Escriva from Valencia, a military architect. The daring hexagonal shape drew fierce criticism from his contemporaries, to such an extent that in 1538 Escriva defended his design in a published Apologia.
In fact, with its double tenaille, numerous embrasures in the bastions and high walls surrounded by a moat, the castle was admirably suited to the topography of the site and the strategic and defensive functions.
It continued to be military property until 1976, when an enormous restoration project was undertaken by the provincial authority of the Provveditorato alle Opere Pubbliche of the Regione Campania. In seven years the original castle was freed of centuries of accretions, and made structurally sound, recreating the original galleries, parapet walkways and underground chambers, where an auditorium seating 700 has been created. In 1982 the site was handed over to the Soprintendenza per i Beni Artistici e Storici of Naples.
The former Marine headquarters now houses the castle administration and some administrative offices for Naples, including the Catalogue Office, Photographic Archives and the Thefts Office. The intention is to make Castle Sant'Elmo a polyfunctional center for activities including contemporary art, the performing arts and congresses.